1. Field of the Invention
The present invention relates generally to a DC/DC converter and specifically to a system and method to efficiently and effectively start-up high power isolated boost DC/DC converters.
2. Discussion of the Prior Art
The need to reduce fossil fuel consumption and emissions in automobiles and other vehicles predominately powered by internal combustion engines (ICEs) is well known. Vehicles powered by electric motors attempt to address these needs.
Typically, a vehicle propelled by an electric motor can use batteries or fuel cells to generate the necessary current. Fuel cells generate electrical power through an electrochemical reaction of a fuel and oxidant, such as hydrogen and oxygen. Water is the product of the electrochemical reaction in a fuel cell utilizing hydrogen and oxygen, a product that is easily disposed. See generally, U.S. Pat. No. 5,991,670 to Mufford.
The desirability of using electric motors to propel a vehicle is clear. There is great potential for reducing vehicle fuel consumption and emissions with no appreciable loss of vehicle performance or drive-ability. Nevertheless, new ways must be developed to optimize these potential benefits.
One such area of electric vehicle (EV) development is converting direct current (DC) generating devices such as fuel cells and batteries for their appropriate load. Ideally, the current generators (such as a high voltage (HV) battery or fuel cell) and loads (such as a vehicle""s 12 volt powered accessories) would all be at the same voltage level. Unfortunately, this is not presently the case. For example, the conventional 12-volt system still exists in an electric vehicle to power the conventional 12-volt loads such as lights, sensors and controllers, while a high voltage bus (for example, 300-V) feeds the traction inverter and motor. There is a dual-voltage power system in the electric vehicle and energy needs to be transferred bi-directionally between the two voltage systems.
Therefore, a successful implementation of electric traction motor propelled vehicles requires an effective bi-directional DC/DC converter. The converter must be bi-directional because the high voltage bus can be used as a current load during start-up or as a current generator. Similarly, the 12-volt battery can be used as a current generator or as a load while charging. DC/DC converters are known in the prior art. Even bi-directional DC/DC converters are known. See generally, U.S. Pat. No. 5,745,351 to Taurand and U.S. Pat. No. 3,986,097 to Woods.
One type of possible isolated full-bridge boost converter that could be useful in an automotive application is one operation mode of a bi-directional DC/DC converter. Isolated full-bridge boost converters are known in the prior art. They boost the voltage from 12-V to about 300-V to start up the fuel cell EV and also provide galvanic isolation between the high voltage bus and 12-V bus. An isolated full-bridge boost converter is also attractive in applications such as single-stage power factor correction (PFC) converters with isolation requirement.
Similar to other current-fed converters, an isolated full-bridge boost converter has several design advantages such as multi-output capability with one shared choke and inherent over-current and short circuit protection. Unfortunately, isolated full-bridge boost converters have not enjoyed widespread application because they require an additional start-up circuit. Further, transformer leakage inductance causes high transient voltage across the bridge switches during commutation transition. Since the circuitry must be able to withstand the voltage spike, high voltage rated switches are required. This makes the circuitry much more expensive. Alternatively, this high transient voltage could be clamped (or damped), thus allowing the use of more inexpensive switches.
Unfortunately, there is no known technology to efficiently and effectively reduce the start-up xe2x80x9cin-rushxe2x80x9d current of a high power isolated boost DC/DC converter. High power converters would be typically more than 1 kw. Such technology would make a converter circuit more practical, cost effective and would facilitate widespread application for the isolated boost converters.
Accordingly, an object of the present invention is to provide an efficient and cost effective high power bi-directional DC/DC converter suitable for automotive applications.
Specifically, the present invention is a scheme for high power isolated full-bridge boost DC/DC converters to minimize the effect of in-rush current during start-up process. A single pulse width modulation controller (PWM) is possible for the present invention for not only start-up but also normal boost modes.
A primary circuit can have a clamping switch or two clamping choke diodes. The clamping diodes can include xe2x80x9cpush-pullxe2x80x9d and xe2x80x9cL-typexe2x80x9d configurations. A resistor or small buck converter can be used to dissipate energy clamped from the voltage spike. A startup circuit can also use the clamping circuit to clamp the voltage spike experienced during start-up.
Since the present invention eliminates the need to match characteristics of multiple controllers, it significantly reduces the cost associated with implementing this type of technology.
Other objects of the present invention will become more apparent to persons having ordinary skill in the art to which the present invention pertains from the following description taken in conjunction with the accompanying figures.